Voltage clamp experiments are carried out to determine the function and structure of ionic channels in single giant nerve fibers. Information is obtained about the position and properties of membrane ionic channel sites that limit conductances and determine ionic selectivity by an analysis of the interaction between current-carrying and blocking ions. The ability of various rate theory models to describe the flow of ions through open potassium channels is tested. Kinetics of ionic blocking of channels are studied to gain information about energy barriers crossed by ions entering channels, and about the nature of ion binding sites within the channels. Analogies between the kinetics of ionic currents across the nerve membrane and those seen in excitable lipid bilayer membrane preparations treated with channel forming agents are investigated. The effect of calcium ions upon sodium channel inactivation is studied.